1. Field of the Invention
The present invention relates to a laser annealing method of, for example, crystallizing an amorphous semiconductor to obtain a polycrystal semiconductor film used for thin film transistors (TFT) formed in a display area and a driver circuit section in an LCD (liquid crystal display) apparatus.
2. Description of the Related Art
In recent years, LCDs have steadily been employed in OA and AV devices because of advantages in their small size and thickness and their low power consumption. In particular, an active matrix type, in which each pixel is equipped with a TFT as a switching element for controlling the rewrite timing of image data, realizes animation display with high resolution on a large screen, and is therefore used for displays in various televisions, personal computers, and the like.
A TFT is a FET (field effect transistor) obtained by forming a semiconductor layer together with a metal layer in a predetermined shape on an insulating substrate. In an active matrix type LCD, a TFT is connected to an electrode of each capacitor as a pixel for driving liquid crystal, formed between a pair of substrates.
In particular, developments have been made to an LCD using a polycrystal silicon (p-Si) as a semiconductor layer in place of amorphous silicon (a-Si) which had previously been common, and annealing with use of a laser beam has been put to use for formation or growth of crystal grains. In general, p-Si has a higher mobility than a-Si so that TFTs can be downsized and a high aperture ratio, by using p-Si to form TFTs, and a high resolution can be realized. In addition, since TFTs can be constructed in a gate self-alignment structure, fine TFT element is achieved and higher speed operation can be achieved by reductions in parasitic capacity. By using these TFTs to form an electric complementary connection structure between an n-ch TFT and a p-ch TFT, i.e., a CMOS, a high speed driver circuit can be constructed. Therefore, a driver circuit section can be formed to be integral with a display area on one same substrate, so that manufacturing costs can be reduced and the LCD module realizes a small size.
As a method of forming a p-Si layer on an insulating substrate, there is a crystallization method by annealing a-Si formed under a low temperature or a solid phase crystallization (SPC) method under a high temperature. In many cases, the treatment must be carried out under a high temperature of 900.degree. C. or more. Therefore, it is not possible to use a low cost non-alkaline glass substrate in view of heat resistance, but a costly quartz glass substrate is required, resulting in a higher manufacturing cost. In contrast, developments have been made to a method which allows use of a non-alkaline glass substrate as an insulating substrate by performing silicon polycrystallization processing at a relatively low temperature of 600.degree. C. or less, with use of laser annealing. The process, in which the processing temperature is 600.degree. C. or less throughout all TFT manufacturing steps is called "low-temperature process", and is necessary for mass-production of LCDs at low cost.
FIG. 1 shows a state of substrate to be processed by excimer laser annealing processing (hereafter referred to as ELA). A substrate 1 to be processed is a popular non-alkaline glass substrate. An a-Si layer is formed on the surface of the substrate 1. An active matrix substrate 5 is a substrate constructing an LCD comprising a display area 2 where display pixels are arranged in matrix, and a gate driver 3 and a drain driver 4 provided in the periphery of the display area 2. The substrate 1 is a mother glass substrate including six active matrix substrates 5. In the display area 2, pixel electrodes, each being an electrode of a pixel capacitor for driving liquid crystal are formed and arranged in matrix, and are respectively connected with TFTs formed. A gate driver 3 is mainly constructed by a shift register, and a drain driver 4 is mainly constructed by a shift register and a sampling circuit. These drivers are formed by a TFT array such as a CMOS or the like. Each TFT is formed in a manner in which p-Si crystallized from a-Si by ELA is used as an operation layer.
As shown in FIG. 1, in a conventional laser annealing method, a line beam is irradiated on a substrate 1 such that the contour of edge lines C of a band-like irradiation region on the substrate 1 is shifted by a predetermined overlap amount. Scanning is carried out as indicated by arrows, and the entire substrate is subjected to annealing. However, after scanning is thus performed with a line beam, there remains a defective crystallization region in which sufficient crystallization was not attained and grains remain with a smaller grain size, as indicated by reference R in the figure, in p-Si formed. This region is formed in a linear shape along the longitudinal direction of the irradiation region, and appears to be a striped pattern. Since this defective crystallization region R has a low mobility and a high resistance, the characteristics of TFTs formed in this region are degraded. If the characteristics of TFTs are thus degraded, pixel capacitors are not sufficiently charged in the display area so that the contrast ratio is lowered or erroneous operation is caused in the peripheral driver circuit section, thus making disadvantageous influences on display.
It is estimated that a defective crystallization region as described above is caused because of unevenness in energy of an irradiated laser beam. Laser annealing strongly depends on the energy of the irradiated laser beam. In general, the grain size of crystalline tends to increase as the irradiation energy increases. However, when the energy level increases to a certain level Eu or more, the grain size rapidly decreases. Hence, it is demanded that the energy level of a laser beam to be irradiated onto an a-Si layer should be as large as possible within a range of Eu to Ed which is lower than an upper limit level Eu such that the energy level does not exceed the upper limit Eu, in order to enlarge the grain size as much as possible thereby to achieve TFTs having excellent characteristics.
FIG. 2 shows an energy distribution of an irradiation beam with respect to positions in a line beam. An optical system for generating a line beam is provided with a line width adjust slit and a line length adjust slit, to form a line beam of a band-like or rectangular shape. Thus, since the line width A of a line beam is defined by the line width adjust slit, the characteristic curve of the irradiation light intensity distribution has substantially sharp edges and a substantially flat energy distribution peak portion Eo, as shown in FIG. 2. However, in regions X and B in FIG. 2, the energy level is extremely high or low and is thus greatly different from the level in the flat portion.
In an optical system comprising a plurality of lenses, light is diffracted or interfered due to slight concave and convex portions existing in the lens surfaces and foreign material contamination or the like sticking thereto. The light thus diffracted or interfered is converged in the line width direction A and is expanded in the line length direction, so that ununiformity of energy of the laser beam irradiated toward the substrate 1 from the optical system is serious. Even if only a slight amount of foreign material or the like exists in a clean room, it may cause unevenness in light intensity and greatly influence energy distribution. Therefore, ununiformity of the output energy of a line beam cannot be completely eliminated at present, and it is unavoidable that the energy level of a line beam to be irradiated partially exceeds the upper limit which allows an appropriate grain size.
As a result of this, a line beam whose energy level is uneven is intermittently irradiated as shown in FIG. 2, and a laser beam which partially exceeds the upper limit Eu of the energy level is irradiated within a unit irradiation region having edge lines C as shown in FIG. 1. For these reasons it is considered that a much finer linear defective crystallization region R is caused within the edge lines C.